1. Field of the Invention
This invention relates to: an oxide evaporation material used when a transparent conducting film is formed by any of various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation; a transparent conducting film formed using the oxide evaporation material; and a solar cell using the transparent conducting film as an electrode. Particularly, the invention relates to an improvement in an oxide evaporation material for forming a high-quality transparent conducting film which is useful as a transparent electrode of a solar cell, has a low resistance, and exhibits a high transmittance in the visible to near-infrared region.
2. Description of the Related Art
A transparent conducting film has a high conductivity and a high transmittance in the visible region. By taking advantage of these characteristics, the transparent conducting film is utilized as an electrode or the like of solar cells, liquid crystal display elements, and various other light receiving elements. Furthermore, by taking advantage of the reflection and absorption characteristics in the near-infrared region, the transparent conducting film is utilized also as: a heat-ray reflection film used for window glasses of automobiles, architectures, and the like; a variety of antistatic films; and an anti-fogging transparent heater for refrigerated showcases or the like.
Generally, the widely used transparent conducting films are formed of: tin oxide (SnO2) containing antimony or fluorine as a dopant; zinc oxide (ZnO) containing aluminum, gallium, indium, or tin as a dopant; indium oxide (In2O3) containing tin, tungsten, or titanium as a dopant; and the like. Particularly, an indium oxide film containing tin as a dopant, i.e., an In2O3—Sn film is referred to as an indium tin oxide (ITO) film, and is industrially widely used to date because a low-resistance transparent conducting film is easily obtained.
As to a method for forming such transparent conducting films, generally used are vacuum deposition methods, sputtering methods, and methods involving application of a coating for forming a transparent conducting layer. Above all, the vacuum deposition methods and the sputtering methods are effective methods for a case where a material having a low vapor pressure is used or where precise film thickness control is required. Moreover, these methods are very simple in operation and thus industrially useful. As the vacuum deposition methods are compared with the sputtering methods, the vacuum deposition methods are capable of forming a film at a faster rate and thus superior in productivity.
In the vacuum deposition methods, generally, a solid or liquid evaporation source is heated in a vacuum of approximately 10−3 to 10−2 Pa and temporarily decomposed to gas molecules or atoms which are then condensed on the surface of a substrate as a thin film again. Among various heating methods for an evaporation source, a resistance heating method (RH method) or an electron-beam heating method (EB method, electron beam deposition) is generally used. A reactive evaporation method is also well known in which a reactive gas such as an O2 gas is introduced into a film-formation chamber (chamber) for deposition.
The electron beam deposition has been historically frequently utilized for depositing an oxide film such as ITO. Specifically, an ITO oxide evaporation material (may also be called an ITO tablet or an ITO pellet) is used as the evaporation source, and an O2 gas serving as the reactive gas is introduced into a film-formation chamber (chamber). Thermal electrons jumped off from a thermal-electron generating filament (mainly a W wire) are accelerated by an electric field and radiated to the ITO oxide evaporation material. The oxide evaporation material is locally heated at the radiated area thereof, and evaporated and deposited to a substrate. Meanwhile, activated reactive evaporation (ARE method) is also a useful method for ITO film formation. In this method, a plasma is generated using a thermal electron emitter or RF discharge, and an evaporation material and a reactive gas (O2 gas, or the like) are activated by this plasma, thereby forming a low-resistance film on a low-temperature substrate. Furthermore, high-density plasma-assist evaporation (HDPE method) using a plasma gun also has been revealed to be an effective method for ITO film formation, and begun to be industrially widely used recently [see “Vacuum,” Vol. 44, No. 4, 2001, pp. 435-439 (hereinafter, “Non-Patent Document 1”)]. This method utilizes an arc discharge using a plasma generator (plasma gun). The arc discharge is maintained between a cathode inside the plasma gun and a crucible (anode) of an evaporation source. Electrons emitted from the cathode are guided by a magnetic field, concentrated and radiated to a local area of an ITO oxide evaporation material put in the crucible. An evaporant is generated from the area that is locally heated by the radiation of the electron beams, and deposited to a substrate. The vaporized evaporant and an introduced O2 gas are activated in this plasma, so that an ITO film having favorable electrical characteristics can be formed. Meanwhile, as another classification of these various vacuum deposition methods, those involving ionization of an evaporation material and a reactive gas are collectively referred to as ion plating (IP method). Ion plating is effective as a method to obtain an ITO film having a low resistance and a high transmittance [see “Technology of transparent conductive film,” Ohrmsha, Ltd., 1999, pp. 205-211 (hereinafter, “Non-Patent Document 2”)].
Meanwhile, in any type of solar cell using a transparent conducting film, the transparent conducting film is essential for an electrode on the front side from which light enters the cell. As the transparent conducting film, the aforementioned ITO film or a zinc oxide (ZnO) film doped with aluminum or gallium has been conventionally utilized. These transparent conducting films are required to have such characteristics as a low resistance and a high transmittance of sunlight. As methods for forming these transparent conducting films, the above-described vacuum deposition methods such as ion plating and high-density plasma-assist evaporation are used.
An oxide evaporation material used in the above-described vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation is a sintered body small in size (for example, having a diameter of approximately 10 to 50 mm, and a height of approximately 10 to 50 mm). This limits the amount of film that can be formed from a single oxide evaporation material. Moreover, when the remaining amount of oxide evaporation material is decreased as the consumed amount is increased, the following procedure has to be performed: terminating the film formation; introducing air into the film-formation chamber in the vacuum state for replacement with a fresh oxide evaporation material yet to be used; and evacuating the film-formation chamber again. This consequently lowers the productivity.
Essential techniques adopted in mass production of transparent conducting films by the vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation include a method of continuously supplying the oxide evaporation materials. Non-Patent Document 1 describes an example of such a continuous supply method. In the continuous supply method, cylindrical oxide evaporation materials are housed in series inside a cylindrical hearth, and are sequentially pushed out and continuously supplied while the height of the sublimation surface is kept the same. The continuous supply method of an oxide evaporation material enables mass production of transparent conducting films by the vacuum deposition methods.
As the oxide evaporation material used as the raw material, Japanese Patent Laid-open Application No. Hei 8-104978 (hereinafter, “Patent Document 1”) proposes an ITO-evaporation material which is substantially In2O3—SnO2 based particles made of indium, tin, and oxygen, having a volume of 0.01 to 0.5 cm3 per particle, a relative density of 55% or more, and a bulk density of 2.5 g/cm3 or lower when filled in a container. Patent Document 1 states that this structure enables manufacturing of an ITO-evaporation material which is capable of stable formation of a low-resistance ITO film by electron beam deposition with a utility efficiency of 80% or more and is continuously suppliable without clogging in a supplying machine.
Meanwhile, as a raw material used in the sputtering methods (i.e., a sputtering target material), various compositions are proposed for an indium oxide-based transparent conducting film other than ITO. For example, Japanese Patent No. 3445891 and Japanese Patent Laid-open Application No. 2005-290458 (hereinafter, respectively “Patent Documents 2 and 3”) each propose techniques related to a sputtering target material made of indium oxide containing cerium (In—Ce—O); and a transparent conducting film obtained by sputtering the sputtering target material. Moreover, Patent Document 2 states that, since the indium oxide-based transparent conducting film containing cerium proposed therein poorly reacts with Ag, a transparent conducting film having a high transmittance and excellent heat resistance can be formed when the indium oxide-based transparent conducting film is stacked on a Ag-based ultra-thin film. Patent Document 3 states that a film having excellent etching characteristics is obtained, and so forth. Furthermore, a crystalline transparent conducting film made of indium oxide containing tungsten (crystalline In—W—O) has been recently revealed to be useful as a transparent electrode of a solar cell [see Japanese Patent Laid-open Application No. 2004-43851 (hereinafter, “Patent Document 4”)].
These indium oxide-transparent conducting films other than ITO are low in resistance, and excellent in transmittance in the visible region. In addition, in transmittance in the near-infrared region, these indium oxide-transparent conducting films are superior to the above-described conventionally-used ITO film and zinc oxide film. The use of such transparent conducting films as an electrode on the front side of a solar cell enables effective utilization of the energy of the near-infrared light.
However, there are few techniques related to an oxide evaporation material for stably forming the above-described indium oxide-transparent conducting films other than ITO by vacuum deposition methods. Despite of considerably high worldwide demands for solar cells in recent years, there are very few techniques related to: an oxide evaporation material from which a transparent conducting film useful as a transparent electrode of a solar cell is effectively formed by deposition methods; and a deposition film formed therefrom.
For this reason, a technique for producing a sintered body of a sputtering target has been also adopted so far for an oxide evaporation material used in vacuum deposition methods. However, in the case of forming a transparent conducting film having a low resistance and a high transmittance by any of various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation, using a conventional oxide evaporation material manufactured by the technique adopted so far, a large amount of oxygen gas needs to be introduced into a film-formation vacuum chamber during the film formation. This brings about problems mainly described below.
First, the transparent conducting film and the oxide evaporation material greatly differ in composition from each other, making it difficult to design the composition of the transparent conducting film. This is because, generally, when a larger amount of oxygen is introduced into a film-formation vacuum chamber, the difference in composition between a transparent conducting film and an oxide evaporation material is likely to increase. In the mass production process of films, the amount of oxygen in a film-formation vacuum chamber also tends to vary. Due to the variation in the oxygen amount, the compositions of the films are likely to differ from one another, resulting in the variation of the film characteristics.
Moreover, when the oxygen amount is increased, film formation by reactive evaporation using an oxygen gas causes problems that not only does the film density decrease, but also the adhesive force of the film to the substrate weakens, for example. These problems occur for the following reason. Specifically, when evaporated metal oxide is oxidized before reaching the substrate, the energy is lost. Thus, an increase in the oxidation ratio makes it difficult to obtain a dense film strongly adhering to the substrate.
Furthermore, suppose a case where a transparent conducting film is formed on a substrate covered with a metal film or an organic film having a surface that can be oxidized easily. In this case, if a large amount of oxygen gas is introduced into a film-formation vacuum chamber, the substrate surface is oxidized before film formation. This hinders fabrication of a high-performance device. This tendency becomes more noticeable as the temperature of the substrate during the film formation is higher. In the case of manufacturing a solar cell that receives light from a surface on the side opposite to the substrate and converts the light into energy, for example, a transparent conducting film needs to be formed on a PIN element formed of metal thin films. Accordingly, if a film is formed with a large amount of oxygen introduced, the element is likely to be damaged, hindering fabrication of a high-performance device. The same is true for forming organic thin-film solar cells and top emission-type organic electroluminescent elements. When a transparent conducting film is formed on an organic light-emitting layer, the organic light-emitting layer is oxidized and thus damaged under a condition where a large amount of oxygen is introduced. Hence, a high-performance element cannot be formed.